Conventional traffic or road signs currently in widespread use are made of a strong metal sheet or backplane supported on a post. The backplane is painted with a background color, while the words and symbols, or informational legend, are painted over the background in contrasting colors. A typical example is a "STOP" sign, which has an octagonal backplane with a red background, a white border around the backplane, and the word "STOP" in white letters. The chosen colors of these road signs are intended to be conspicuous so that the signs are easily noticed by drivers, while the color combinations are highly contrasting so that the information can be easily discernible from a distance. In addition, the paints used on the signs are substantially reflective such that at night, when they are directly illuminated by approaching headlights, they reflect much of the light back to the drivers. Therefore, the signs should theoretically be highly legible in the dark.
In practice, the readability of conventional signs depend upon the actual viewing conditions. The signs are adequately legible when their front faces are illuminated by frontal or direct lighting during the day, and direct lighting by headlights during the night. However, if they are predominately illuminated from the back, or backlit, by the sun, a bright haze, or oncoming headlights, the areas surrounding the signs can appear very bright, while the faces of the signs can appear so dark that they can become unreadable.
Some signs have been designed to alleviate this back lighting problem. U.S. Pat. No. 5,050,327 to Woltman (1991) shows a sign with legend segments or letters which are mostly retroreflective, but somewhat translucent. Under direct lighting, the legend will reflect much of the light such that it will be highly visible. On the other hand, when the sign is backlit by the sun or oncoming headlights, the slightly translucent legend will allow some of the light to pass through so as to make the legend glow against the dark background of the sign. The glowing message will allow the sign to be more readily readable when backlit. However, utilizing the retroreflectivity and translucency of the same material has a tradeoff. The more retroreflective the material, the less translucent it is, and vice versa. Therefore, the readability of the sign will be highly compromised, such that it will either be very readable in direct lighting, but not in back lighting, or it will be very readable in back lighting, but not in direct lighting.
U.S. Pat. No. 4,846,549 to Gutsche (1989) shows a display device, in FIG. 3, which has entirely translucent legend segments. When strongly backlit, lenses behind the translucent segments focus light onto the segments as bright spots, such that the segments appear as lines of bright dots. However, this sign will only work in this manner if the back lighting occurs at almost normal to the plane of the sign, otherwise the focal points of the lenses will not fall onto the translucent segments. Moreover, the purely translucent segments will reflect little light, such that they will be very difficult to read in direct or oblique lighting.
In conclusion, no existing sign is highly readable in direct, oblique, and back lighting conditions.